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8.01 Classical Mechanics Chapter 3

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8.01 Classical Mechanics Chapter 3 Chapter 3 Vectors 3.1 Vector Analysis ....................................................................................................... 1 3.1.1 Introduction to Vectors ................................................................................... 1 3.1.2 Properties of Vectors ....................................................................................... 1 3.2 Coordinate Systems ................................................................................................ 6 3.2.1 Cartesian Coordinate System ......................................................................... 6 3.2.2 Cylindrical Coordinate System ....................................................................... 8 3.3 Vectors ................................................................................................................... 10 3.3.1 The Use of Vectors in Physics ....................................................................... 10 3.3.2 Vectors in Cartesian Coordinates ................................................................. 11 Example 3.1 Vector Addition ................................................................................. 14 Example 3.2 Sinking Sailboat ................................................................................ 14 Example 3.3 Vector Addition ................................................................................. 16 Example 3.4 Vector Description of a Point on a Line .......................................... 18 3.3.2 Transformation of Vectors in Rotated Coordinate Systems ...................... 19 Example 3.5 Vector Decomposition in Rotated Coordinate Systems ................. 21 3.4 Vector Product (Cross Product) .......................................................................... 23 3.4.1 Right-hand Rule for the Direction of Vector Product ................................ 23 3.4.2 Properties of the Vector Product .................................................................. 25 3.4.3 Vector Decomposition and the Vector Product: Cartesian Coordinates .. 25 3.4.4 Vector Decomposition and the Vector Product: Cylindrical Coordinates 27 Example 3.6 Vector Products ................................................................................. 27 Example 3.7 Law of Sines ....................................................................................... 28 Example 3.8 Unit Normal ....................................................................................... 28 Example 3.9 Volume of Parallelepiped ................................................................. 29 Example 3.10 Vector Decomposition ..................................................................... 29 Chapter 3 Vectors Philosophy is written in this grand book, the universe which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles and other geometric figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth.1 Galileo Galilee 3.1 Vector Analysis 3.1.1 Introduction to Vectors Certain physical quantities such as mass or the absolute temperature at some point in space only have magnitude. A single number can represent each of these quantities, with appropriate units, which are called scalar quantities. There are, however, other physical quantities that have both magnitude and direction. Force is an example of a quantity that has both direction and magnitude (strength). Three numbers are needed to represent the magnitude and direction of a vector quantity in a three dimensional space. These quantities are called vector quantities. Vector quantities also satisfy two distinct operations, vector addition and multiplication of a vector by a scalar. We can add two forces together and the sum of the forces must satisfy the rule for vector addition. We can multiply a force by a scalar thus increasing or decreasing its strength. Position, displacement, velocity, acceleration, force, and momentum are all physical quantities that can be represented mathematically by vectors. The set of vectors and the two operations form what is called a vector space. There are many types of vector spaces but we shall restrict our attention to the very familiar type of vector space in three dimensions that most students have encountered in their mathematical courses. We shall begin our discussion by defining what we mean by a vector in three dimensional space, and the rules for the operations of vector addition and multiplication of a vector by a scalar. 3.1.2 Properties of Vectors A vector is a quantity that has both direction and magnitude. Let a vector be denoted by the symbol A . The magnitude of A is | A | A . We can represent vectors as geometric ≡ objects using arrows. The length of the arrow corresponds to the magnitude of the vector. The arrow points in the direction of the vector (Figure 3.1). 1 Galileo Galilei, The Assayer, tr. Stillman Drake (1957), Discoveries and Opinions of Galileo pp. 237-8. 1 Figure 3.1 Vectors as arrows. There are two defining operations for vectors: (1) Vector Addition: Vectors can be added. Let A and B be two vectors. We define a new vector, C = A + B , the “vector addition” of A and B , by a geometric construction. Draw the arrow that represents A . Place the tail of the arrow that represents B at the tip of the arrow for A as shown in Figure 3.2a. The arrow that starts at the tail of A and goes to the tip of B is defined to be the “vector addition” C A B . There is an equivalent construction for the = + law of vector addition. The vectors A and B can be drawn with their tails at the same point. The two vectors form the sides of a parallelogram. The diagonal of the parallelogram corresponds to the vector C = A + B , as shown in Figure 3.2b. C = A + B C = A + B B B A A (a) head to tail (b) parallelogram Figure 3.2a Figure 3.2b Vector addition satisfies the following four properties: (i) Commutativity: The order of adding vectors does not matter; A + B = B + A . (3.1.1) Our geometric definition for vector addition satisfies the commutative property (3.1.1). We can understand this geometrically because in the head to tail representation for the 2 addition of vectors, it doesn’t matter which vector you begin with, the sum is the same vector, as seen in Figure 3.3. C = A + B C = B + A B A B A Figure 3.3 Commutative property of vector addition. (ii) Associativity: When adding three vectors, it doesn’t matter which two you start with ( A B) C A ( B C) . (3.1.2) + + = + + In Figure 3.4a, we add ( B + C ) + A , and use commutativity to get A + ( B + C ) . In figure, we add ( A B ) C to arrive at the same vector as in Figure 3.4a. + + C A A + (B + C) (A + B) + C B B B + C A + B A C Figure 3.4a Associative law. (iii) Identity Element for Vector Addition: There is a unique vector, 0 , that acts as an identity element for vector addition. For all vectors A , A + 0 = 0 + A = A . (3.1.3) (iv) Inverse Element for Vector Addition: For every vector A , there is a unique inverse vector −A such that 3 A ( A ) 0 . (3.1.4) + − = The vector −A has the same magnitude as A , | A |= | −A |= A , but they point in opposite directions (Figure 3.5). A A Figure 3.5 Additive inverse (2) Scalar Multiplication of Vectors: Vectors can be multiplied by real numbers. Let A be a vector. Let c be a real positive number. Then the multiplication of A by c is a new vector, which we denote by the symbol c A . The magnitude of c A is c times the magnitude of A (Figure 3.6a), cA = c A . (3.1.5) Let c > 0 , then the direction of c A is the same as the direction of A . However, the direction of −c A is opposite of A (Figure 3.6). A c A c A Figure 3.6 Multiplication of vector A by c > 0 , and −c < 0 . Scalar multiplication of vectors satisfies the following properties: (i) Associative Law for Scalar Multiplication: The order of multiplying numbers is doesn’t matter. Let b and c be real numbers. Then b( cA ) = (bc )A = (cb A) = c( b A) . (3.1.6) (ii) Distributive Law for Vector Addition: Vectors satisfy a distributive law for vector addition. Let c be a real number. Then 4 c( A B ) c A c B . (3.1.7) + = + Figure 3.7 illustrates this property. c(A + B) c A + cB C = A + B cB c A B A Figure 3.7 Distributive Law for vector addition. (iii) Distributive Law for Scalar Addition: Vectors also satisfy a distributive law for scalar addition. Let b and c be real numbers. Then (b c) A b A c A (3.1.8) + = + Our geometric definition of vector addition and scalar multiplication satisfies this condition as seen in Figure 3.8. (b + c) A bA + c A c A A bA Figure 3.8 Distributive law for scalar multiplication. (iv) Identity Element for Scalar Multiplication: The number 1 acts as an identity element for multiplication, 1 A = A . (3.1.9) 5 Unit vector: Dividing a vector by its magnitude results in a vector of unit length which we denote with a caret symbol A Aˆ = . (3.1.10) A Note that Aˆ = A / A = 1. 3.2 Coordinate Systems Physics involve the study of phenomena that we observe in the world. In order to connect the phenomena to mathematics we begin by introducing the concept of a coordinate system. A coordinate system consists of four basic elements: (1) Choice of origin (2) Choice of axes (3) Choice of positive direction for each axis (4) Choice of unit vectors
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